Service

What is an alternator and how does it work?

The alternator, which is bolted to the engine, keeps your car’s battery charged and produces the electrical power necessary to run the vehicle’s electrical systems. When starting your car, stored energy in the battery powers the starter motor. Once the engine is running, the alternator produces the electrical energy that your car needs by spinning a magnet on a rotor through a stationary coil of wire. To ensure that the alternator provides the right voltage output for your car’s electrical system, there is a voltage regulator built into the alternator. The voltage regulation is built into the car’s Powertrain Control Module (PCM) on newer vehicles.

Alternator

When to replace the alternator?

The following electrical or mechanical faults are signs that the alternator may need to be replaced:

Battery or charging system warning light. If a warning light has illuminated, the charging system and the vehicle battery should be tested to pinpoint the fault.

Difficulty starting your car or it doesn’t start. A faulty alternator will lead to the battery being discharged. Once the battery has discharged enough, you will have difficulty starting your car or it won’t start at all.

Dimming lights. Should the alternator have an internal electrical fault, such as a burned-out diode or open stator windings, the alternator will have less capacity to provide electrical power.

Grinding or whining noise. High heat created by the engine dries out the grease in the alternator bearings, wear occurs, and then the alternator can emit a grinding or whining noise as it spins. Certain electrical failures within an alternator can also cause a whining noise.

Premature Battery Failure. If the voltage regulator within the alternator has failed, and thus charging system output is too high, that can cause the battery to fail due to overcharging. Although this fault is not as common as a weak alternator, the possibility should always be considered if your car battery fails prematurely.

On an annual basis, or every 12,000 miles. The voltage output of the alternator should be carefully measured and compared to the OEM factory specification. Voltage output should also be checked whenever the vehicle battery is replaced.

How do mechanics replace an alternator?

Disconnect the battery prior to replacing the alternator, as with all electrical repairs.

Unhook the drive belt connected to the alternator.

The electrical connections to the alternator are removed and the alternator is unbolted from its cradle on the engine.

The new alternator is installed while observing all factory bolt torque specifications. Alternators have to be mounted to the engine very securely because they rotate at high speed. Electrical connections are cleaned and re-attached, including the connections to the battery.

While the drive belt is off, best practice is to inspect the drive belt as well as inspect all pulleys driven by the belt.

Once all components are securely in place, the vehicle engine is run and the output of the new alternator is measured and verified against the OEM factory specification.

Is it safe to drive with an alternator problem?

One should exercise caution. Should the alternator fail while you are driving, the battery alone will power the car’s electrical system for a short while. Cars require a substantial amount of electrical power though and the battery will likely discharge relatively quickly. The car may stall and it would be impossible to re-start if the battery discharged completely. When the battery or charging system warning light illuminates, the situation should be investigated and resolved as soon as possible.

When replacing the alternator keep in mind:

An alternator should always be tested to be sure it is actually faulty versus a circumstance where faulty electrical connections, or an open fuse, might be the problem.

An alternator that has noisy or worn out bearings should be replaced regardless of electrical test results.

A properly functioning battery is very important in modern cars and consequently the battery should be load tested when the alternator is replaced.

Alternators are designed to power all vehicle electronics and maintain a fully charged battery but if the battery has failed and won’t hold a charge, attempting to continually re-charge such a battery by driving can overload the alternator. Replace the battery and avoid repeatedly jump-starting your car.

Electrical connections, including grounds, are very important in the charging system. When replacing the alternator, all disconnected terminals should be inspected and cleaned and then checked to be sure all connections are properly tightened.